The various objectives proposed comprise a broad program designed further to elucidate, on the basis of experiments to be conducted upon inbred strains of a variety of rodent species, both the genetic and other requirements for the same elicitation of allograft immunity and the manner in which it is enabled to express itself in the phenomena of allograft reactions and graft-versus-host reactions in vivo, and in their in vitro correlates such as the Mixed Leukocyte Reaction and Cell-Mediated Cytotoxicity. Special emphasis will be placed upon the cell-mediated component of the host response to allografts. Attention will also be paid to the definition of processes of a regulatory nature that, under certain conditions, either impair the development of these reactions or interfere with their fulfillment. Extensive use will be made of the Syrian hamster; the immunologic response aberrations of this species appear to be related to genetic defects at the major histocompatibility complex and may serve as a natural experiment illuminating the biologic role of this important chromosomal region in other species. The work includes further studies upon the uterus as a unique graft site, and upon the feto-placental unit as an allograft. It is planned further to document and establish the immunological significance to the natural exchanges of cells that take place between mother and fetus before birth. An extensive study will also be undertaken of the significance of the normal transfer of viable leukocytes, including lymphocytes, from mother to neonate via the milk.